Aerospace vehicles such as aircraft are susceptible to lightning strikes and other high intensity radiated fields (HIRF), or collectively voltage spikes or energy spikes. Voltage spikes and induced surges have the potential of interrupting the operation of electrical and control systems within the vehicles. In low-impedance systems, for example in power wiring, induced surges become high-current surges which can trip circuit breakers off-line and disrupt airplane services. In high-impedance systems, for example electronics, induced high-voltage spikes can trip logic, and damage semiconductor avionics. Current generations of aircraft use multiple low-voltage microprocessors, semiconductor devices, and high-frequency data busses, all of which are sensitive to voltage spikes. To mitigate these effects, protection in the form of shielding is used.
For example, in present airplanes with metal fuselages, and especially those produced in last 20 years, at least 90% of the protection required is achieved through the use of metallic shields on critical wiring and cable bundles. The demonstrated best-practice for such shielding (see e.g., “Lightning Protection of Aircraft”, Lightning Technologies Inc., Fisher, 2004 (LTI), Ch. 15, FIG. 15.1) is a copper-braid tube wrap on the entire bundle, terminated at each end by a bonded-ring to the connector back-shell, or other grounding methods depending on each individual case (see e.g., LTI, Ch.15, FIG. 15.23.) While shielding has been proven to work quite well in metal airplanes by reducing the external effects by about 6 dB, it still leaves equipment exposed to 1500V spikes and 3000 Amp current surges (see Standards defined in “Environmental Conditions and Test Procedures for Airborne Equipment”, RTCA-DO-160E, RTCA Incorporated, 2007 (RTCA-DO-160E), Section 22, 23.) Because of these exposures, Line Replaceable Units (LRUs) typically include levels of internal protection to prevent damage, at extra cost and weight. Skilled workmanship is necessary to design and install copper-braided bundle-shields, and during their lifetime end-terminations are exposed to temperature-stress, current surges, and work-hardening breakages due to cable flexing. Special certification procedures are required for cable-shielding to demonstrate effectiveness to the FAA. Also, life expectancy has to be proven to the FAA, as shields are prone to coming loose and breakages are common.
Transformers used for Transformer-Rectifier 28 Vdc Units (TRUs) do provide some isolation, due in part because the secondary is not connected to the primary, but the isolation is nominal and provides only about −6 dB for the 400 Hz due to the 4:1 turns ratio. This protection is deemed acceptable for metal airplanes under RTCA-DO-160E design rules. Other traditional terrestrial solutions such as metal-oxide varistors (MOVs), diodes etc, have not been used mainly because they are not fault-tolerant, and a single latent-failure renders them useless for airplane purposes.
These solutions serve to mitigate the damage to electronics once a voltage spike is present in the vehicle, but do not prevent the voltage spike from entering the vehicle itself. Many fuselages of aircraft are constructed of metal, which provides some protection to the internal wiring and systems by inhibiting the flow of charge from outside into the enclosed metal fuselage. An enclosed metal structure is sometimes referred to as a “Faraday Cage.” In some vehicles, an additional enclosed metal compartment is created within the fuselage to further house and protect flight essential electronics and electrical systems from voltage spikes. However, a recent trend in modern aircraft is to use composite and other non-metal materials, in lieu of metal, in the construction of the vehicle. While these composite materials offer significant reductions in weight, and permit the use of advanced molding methods to achieve perfect aerodynamic forms not previously possible with metal-forming, they also significantly increase risk of damage from electromagnetic fields such as airport radars, high-power radio and TV transmitters Composite materials reduce the beneficial “Faraday Cage” effect of the fuselage, increasing the importance of using other means to prevent voltage spikes from harming the internal systems.
In terrestrial applications, electrical isolation is achieved through transorbs, spark gaps, gas tubes, and transformer isolation. For example, transformers having large volumes of dielectric liquid, or large air gaps, can be used as isolation transformers because there are generally no significant space or weight restrictions. Further, transorbs or components that deteriorate over a number of uses can be easily replaced in terrestrial environments. However, in an aerospace vehicle, there are significant space and weight considerations, and components whose performance deteriorates after every use must be periodically inspected and/or replaced, increasing maintenance time and costs.